Disregard the instructions on the box and just mix the entire can of soda and the cake mix together. Bake for the time it says on the box (depends on the pan you're using, I usually make cupcakes to help limit portion size).

Frost with sugar-free cool whip once cool.

One cupcake will come out to roughly 180 calories (if you only make 12 cupcakes which are pretty big, if you make more it will be less calories per cake).